应该是O（n）呀，难道是常数爆炸了？

#include<bits/stdc++.h>
using namespace std;
const int maxn=20000001;
std::bitset<maxn> prime;
int n,d[maxn],lst[maxn],f[maxn];
signed main()
{
	ios::sync_with_stdio(0);
	cin>>n;
	for(int i=1;i<=n;i++) d[i]=1+log10(i)+d[i-1],prime[i]=true;
    f[1]=1,prime[1]=false;
    for (int i = 2; i <= n; i++) {
        if (prime[i]) {
            lst[++lst[0]] = i;
            f[i]=i-1;
        }
        for (int j = 1; j <= lst[0]; j++) {
            if (i * lst[j] > n) {
                break;
            }
            prime[i * lst[j]] = false;
            if (i % lst[j] == 0) {
                f[i*lst[j]]=f[i]*lst[j];
                break;
            }
            f[i*lst[j]]=f[i]*f[lst[j]];
        }
    }
    double ans=0;
    for(int i=1;i<=n;i++)
    {
    	ans+=f[i]*(d[n/i]+(1+int(log10(i)))*log2(1+int(log10(i))));
	}
	printf("%.6f",ans);
	return 0;
}